1. Field of the Invention
This invention relates to a toner for developing an electrostatic image, used in image forming processes such as electrophotography, electromagnetic recording or electrostatic printing, and an image forming method making use of such a toner.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 and No. 43-24748 and so forth are conventionally known in electrophotography. In general, images are obtained by forming an electrostatic image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic image by the use of a toner to form a toner image, and transferring the toner image to a recording medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure or solvent vapor. Thus, the toner image is fixed on the recording medium to form a fixed image.
Various methods or apparatus have been developed for the above final step, i.e., the step of fixing a toner image to a recording medium such as paper. A method most commonly used at present employs a pressure heating system making use of a heating roller or a stationary heating element provided through a heat-resistant film.
In the pressure heating system making use of a heating roller, fixing is carried out by causing a recording medium toner-fixing sheet to pass through a heating roller whose surface is formed of a material having releasability to toner while the surface of the recording medium, on which a toner image is formed, is brought into contact with the surface of the heating roller under pressure. Since in this method the surface of the heating roller comes into contact with the toner image on the recording medium under pressure, a very good thermal efficiency can be achieved when the toner image is fixed onto the recording medium, so that fixing can be carried out rapidly.
In this method, since the surface of the heating roller comes into contact with the toner image under pressure in the molten state, part of the toner image may adhere and transfer to the surface of the fixing roller, which may re-transfer to subsequent recording medium to cause an "offset phenomenon", and to get the recording medium dirty. This phenomenon is greatly affected by fixing speed and fixing temperature. In general, in the case of a low fixing speed, the surface temperature of the heating roller is set relatively low, and, in the case of a high fixing speed, the surface temperature of the heating roller is set relatively high. This intends that a quantity of heat supplied from the heating roller to the toner in order to fix the toner can be substantially kept constant independent of the fixing speed.
The toner on the recording medium forms several toner layers. In particular, especially in a system where the fixing speed is high and the heating roller has a high surface temperature, there is a great difference in temperature between the upper-most toner layer coming in contact with the heating roller and the lower-most toner layer coming in contact with the recording medium. Therefore, when the heating roller has a high surface temperature, the toner of the upper-most layer tends to cause the offset phenomenon and, when the heating roller has a low surface temperature, the toner of the lower-most layer does not melt well and hence the toner tends to be not fixed to the recording medium to cause a phenomenon called "low-temperature offset".
As a means for solving this problem, in high-speed fixing, a method is conventionally used in which the pressure at the time of fixing is made higher to anchor the toner to the recording medium. This method enables the heating roller temperature to lower to a certain level, inhibiting high-temperature offset of the upper-most toner layer. However, the shear force applied to the toner is so great that the recording medium may wind around the fixing roller to cause wind-around offset, or separation marks of separating claws for separating the recording medium from the fixing roller tend to appear on fixed images. In addition, because of high pressure, line images tend to be crushed at the time of fixing or the toner may scatter to damage the image quality of fixed images.
In the high-speed fixing, it is common to use a toner having a lower melt viscosity than in the case of low-speed fixing and to make the heating roller operate at a lower surface temperature and at a lower fixing pressure so that toner images can be fixed while preventing the high-temperature offset and wind-around offset. However, if such a toner having a low melt viscosity is used in low-speed fixing, the offset phenomenon tends to occur at high temperature.
It is sought to provide a toner which has a fixing temperature range broad enough to be applicable even in high-speed fixing as well as in low-speed fixing, superior anti-offset properties, and good fixing performance for various types of transfer-receiving papers inclusive of reclaimed paper.
As toners are made to have smaller particle diameters, the resolution and definition (or sharpness) of images are improved, but on the other hand the fixing performance may lower at halftone areas formed of the toner having smaller particle diameters. This phenomenon is remarkable especially in the high-speed fixing. This is because the toner is laid in a smaller amount at the halftone areas and hence the toner transferred to concave areas of the recording medium is supplied with heat in a smaller quantity from the heating roller, and also because the pressure applied to the concave areas is held back by convex areas. The toner transferred to the convex areas of the recording medium at the halftone areas is so thin in its layer thickness that the shear force applied to the toner per each particle is larger than that at solid black areas having a large toner layer thickness, so that the offset phenomenon tends to occur and the fixed images tend to have a low image quality.
Japanese Patent Application Laid-open No. 1-128071 discloses a toner for electrophotography, employing polyester resin as a binder resin and having a specific storage elastic modulus at 95.degree. C., but it still needs to be improved in fixing performance and anti-offset properties.
Japanese Patent Application Laid-open No. 4-353866 discloses a toner for electrophotography, having such rheological characteristics that its storage elastic modulus begins to fall at a temperature ranging from 100 to 110.degree. C., a specific storage elastic modulus is held at 150.degree. C. and the peak temperature of its loss elastic modulus is 125.degree. C. or above. However, both the storage elastic modulus and the loss elastic modulus are both too small and also the peak temperature of the loss elastic modulus is too high to improve low-temperature fixing performance, and the storage elastic modulus and the loss elastic modulus are both too low to achieve a high thermal resistance.
Japanese Patent Application Laid-open No. 6-59504 discloses a toner for electrophotography, employing as a binder resin a polyester resin having a specific structure, a specific storage elastic modulus at 70 to 120.degree. C. and a specific loss elastic modulus at 130 to 180.degree. C. However, the storage elastic modulus at 70 to 120.degree. C. is so large, and the loss elastic modulus at 130 to 180.degree. C. is so small, that the toner is difficult to fix at low temperatures when it is a magnetic toner having small particle diameters, and also its anti-offset properties need to be improved.
Japanese Patent Application Laid-open No. 7-224103 discloses a process for producing a high-molecular weight resin used as a resin for toners, employing a polymerization initiator having two peroxide groups in a molecule. However, since only a high-molecular weight resin can be produced, the low-temperature fixing performance of the toner is not improved.
Macromolecule Papers, Vol. 46 (2), pp.81-87, reports synthesis of fluorine-containing block copolymer using a polymeric peroxide. Polymer Journal 24,971 (1992) reports reaction mechanism of radical polymerization initiator having two peroxide groups in a molecule.
In an instance where magnetic toners having small particle diameters contain a magnetic material in a large quantity, the problem on fixing performance is remarkable. From the viewpoint of rheology, an increase of a colorant contained in the toner tends to cause an increase in storage elastic modulus and loss elastic modulus, and the fixing performance may become poor depending on the paper used, thus an improvement is desired.
Japanese Patent Application Laid-open No. 8-234480 (corresponding to EP-A 0718703) discloses a toner in which storage elastic modulus at a temperature of 100.degree. C. (G'.sub.100) and the ratio of storage elastic modulus at a temperature of 60.degree. C. (G'.sub.60) to storage elastic modulus at a temperature of 70.degree. C. (G'.sub.70), G'.sub.60 /G'.sub.70, are specified.
This toner, on account of its viscoelastic properties, rapidly melts when heat is supplied from a fixing assembly at the time of fixing in an amount not less than a predetermined amount, and is firmly anchored to and fixed on a transfer-receiving medium. With regard to anti-blocking properties, it has a sufficiently high storage elastic modulus even at a temperature near to the glass transition temperature (Tg) measured using a differential scanning calorimeter (DSC), and hence, rarely agglomerates and deforms when left for a long time in a high-temperature environment. Thus, it has superior fixing performance and anti-blocking properties and also can achieve superior fixing performance even in such a state that in the cold season a lower roller is not well heated immediately after turning on a switch. However, when images are fixed on thick recording mediums such as a transfer-receiving paper of 80 g/m.sup.2 or 120 g/m.sup.2, heat from the lower roller (pressure roller) is absorbed by the recording medium. Hence, especially when images are fixed under fixing conditions of a high fixing speed, heat is not sufficiently and uniformly imparted to the toner, tending to cause a lowering of fixing performance and a decrease in the image density of fixed images. Thus, there is room for further improvement.